Heat transfer systems in thermal communication with a power source have been utilized to regulate heat generated during the operation of the power source. For example, automotive vehicles have employed heat transfer fluids and cooling systems that transfer and dissipate heat generated as a by-product of gasoline powered internal combustion engines. In this case, the heat transfer fluids and cooling systems ensure that the engine operates in an optimum environment and is not subject to undesirably high temperatures.
However, alternatives to traditional gasoline powered internal combustion engine are now desired, especially alternatives that address public concerns regarding the environment and the management of natural resources. As a result, new power source technologies continue to be developed, especially those that provide improvements in energy efficiency. Examples of alternative power sources that have been developed include, but are not limited to, batteries, fuel cells, solar (or photovoltaic) and internal combustion engines powered by the condensation of steam, natural gas, diesel, hydrogen, and/or the like. Such alternative power sources may be used alone or in combinations thereof, such as those employed in hybrid vehicles.
Although such alternative power sources often provide improvements in energy efficiency as compared to gasoline powered internal combustion engines, they continue to require the use of heat transfer systems and heat transfer fluids. In particular, heat transfer systems and fluids are necessary to maintain optimum operating conditions, particularly in regards to temperature.
Unfortunately, however, traditional prior art cooling systems and heat transfer fluids are unsuitable (or not optimized) for use with alternative power sources, especially those employing electricity or an electrical charge. For example, traditional prior art heat transfer fluids are typically characterized by extremely high conductivities, often in the range of 3000 μS/cm or more. The use of highly conductive heat transfer fluids with alternative power sources, especially electricity based alternative power sources, can result in electrical shock, increased corrosion and/or the short-circuiting of electrical current.
As a result, conventional heat transfer fluids are unsuitable for use with alternative power sources; especially electricity based alternative power sources.
Fuel cells are a particularly attractive alternative power source because of their clean and efficient operation. Fuel cells have been proposed for use in numerous applications.
For example, it has been proposed that fuel cells replace the internal combustion engines currently used in automobiles. Several different kinds of fuel cells are currently under development and appear to hold promise for use in automotive applications. Illustrative examples include Proton Exchange Membrane or Polymer Electrolyte Membrane (PEM) fuel cells, phosphoric acid (PA) fuel cells, molten carbonate (MC) fuel cells, solid oxide (SO) fuel cells, and alkaline fuel cells.
A fuel cell assembly typically comprises an anode, a cathode, and an electrolyte in between the two electrodes. Normally, an oxidation reaction (e.g., H2→2H++2e) takes place at the anode and a reduction reaction (e.g., O2+2H2O+4e→4OH−) takes place at the cathode. The electrochemical reactions that occur at the electrodes are exothermic, i.e., they produce heat.
The successful replacement of internal combustion engines with fuel cells requires that optimal operating conditions be achieved and maintained, i.e., a fuel cell must achieve the desirable current density level without degradation of fuel cell components. It is therefore necessary to control the exothermic heat produced during the electrochemical reactions.
For example, to achieve optimal operating conditions, the normal operating temperature of a PEM fuel cell assembly is controlled so that it remains within a range of from 60° C. to 95° C. Because of the exothermic nature of the electrochemical reactions, it is desirable to use a heat transfer fluid or heat transfer fluid to keep the electrode assembly at an operating temperature that is within the desired operating temperature range. However, the presence of an electrical charge makes it challenging to use fuel cells with prior art heat transfer systems and fluids.
Moreover, in order to produce sufficient power, a fuel cell based automotive engine might have many fuel cells connected together in series to form a fuel cell stack. Individual fuel cells may have an operating voltage of from 0.6 to 1.0V DC. In one instance, it is contemplated that anywhere from 100 to 600 individual fuel cells might be connected in series. As a result, the DC electrical voltage across automotive fuel cell stacks could be very high, typically ranging from 125 to 450 V DC.
These same voltages are experienced in the heat transfer fluid systems of the individual fuel cells used in automotive fuel cell stacks. To prevent or minimize electrical shock hazard, the heat transfer fluid must have very low conductivity. Low electrical conductivity for fuel cell heat transfer fluid is also desirable for the reduction of shunt current in the heat transfer fluid system and the minimization of system efficiency reduction.
There is therefore a need to provide ‘low conductivity’ heat transfer fluids intended for use in heat transfer systems that are in thermal communication with alternative power sources.
In addition to low electrical conductivity, heat transfer fluids used with alternative power sources must also have high heat capacity, low viscosity, and high thermal conductivity. Such properties help minimize pressure drops and reduce pumping power requirements while still meeting heat transfer requirements. Good surface wetting properties are also desirable in a heat transfer fluid employed with alternative power sources. A heat transfer fluid with good surface wetting characteristics is helpful in reducing pressure drops at a condition of constant flow rate.
Another important characteristic of a desirable heat transfer fluid is corrosion resistance. Many heat transfer fluid systems used with alternative power sources often have several metallic components. Illustrative metals found in heat transfer systems employed with alternative power sources include ferrous and non ferrous alloys such as stainless steel, aluminum, brass, braze alloy, and the like. However, such metals are vulnerable to corrosion as a result of contact with the heat transfer fluid.
There is therefore a need to provide corrosion inhibiting heat transfer fluids in heat transfer systems used with alternative power sources that minimize corrosion and prolong the service life of the heat transfer system. More particularly, there remains a need for low conductivity heat transfer fluids that inhibit the corrosion of heat transfer systems in thermal communication with alternative power sources.
Various methods for maintaining low electrical conductivity in a heat transfer fluid have been proposed. For example, WO 00/17951 proposes the use of an ion exchange resin unit to maintain adequate purity of a pure glycol and water heat transfer fluid mixture in a fuel cell system. CA 2 435 593 discloses a method for deionizing a heat transfer medium of a fuel cell utilizing a two heat transfer circuit arrangement and a deionization cell wherein a diluate flows in one heat transfer circuit flowing through a fuel cell stack and a concentrate flow can be part of a secondary heat transfer circuit.
Fuel cell coolants must also have high heat capacity, low viscosity, and high thermal conductivity. Such properties help minimize pressure drops and reduce pumping power requirements while still meeting heat transfer requirements. Good surface wetting properties are also desirable in a fuel cell coolant. A heat transfer fluid with good surface wetting characteristics is helpful in reducing pressure drops at a condition of constant flow rate.
Another important characteristic of a desirable heat transfer fluid is corrosion resistance. Heat transfer systems often have several metallic components. Illustrative metals found in fuel cell cooling systems and other heat transfer systems include ferrous and non ferrous alloys such as stainless steel, aluminum, brass, braze alloy, and the like. However, such metals are vulnerable to corrosion as a result of contact with the heat transfer fluid.
There is therefore a need to provide corrosion inhibiting heat transfer fluids that minimize corrosion of metallic heat transfer system components and prolong the service life of fuel cell cooling systems and other heat transfer systems.
However, many of the corrosion inhibitors previously known for use in internal combustion engine coolants are unsuitable for use in fuel cell heat transfer fluids because they are typically highly conductive ionic species. Illustrative examples of such corrosion inhibitors are silicates, nitrites, molybdates, nitrates, carboxylates, phosphates, borates, and the like. Such ionic corrosion inhibitors cannot be used in fuel cell heat transfer fluids because of the requirement that fuel cell heat transfer fluids have very low conductivity. One major drawback of ion exchange resins or electrodeionization cell methods is that they may remove corrosion inhibitors. As a result, the fuel cell heat transfer fluid may lose its ability to inhibit the corrosion of metal components of the fuel cell heat transfer system.
As a result, the prior art has failed to provide an effective resolution to problems associated with the maintenance of low conductivity in corrosion inhibiting heat transfer fluids for assemblies comprising alternative power sources such as fuel cells.